Maybe lower the apeture to let more light in...I would assume that the shutter speed is too fast for the apeture size (f.stop is too high) so it gets darker. Think about it, at the same apeture(f.#), if you speed up your shutter, less light goes in, thus making the picture appear darker

Hmmm, it's set to my lowest F stop (2.8) but at any shutter speed over 1/20 it seems to significantly dim the image.